This proposal seeks to establish the new scientific paradigm that RNA molecules can function as hormones, traveling via the bloodstream to target organs where they are taken up and influence the activity of specific recipient cells. The conventional paradigm of the endocrine system does not include RNA molecules as a class of hormones. The motivation for trying to establish this paradigm comes from a series of facts: (i) RNA molecules have been shown to function as hormones in plants, (ii) in some animal species (e.g., worms and flies) RNA has been shown to spread throughout the organism and carry information from one site to another, and (iii) we and others have found that at least one class of RNAs, known as microRNAs, are abundantly present in the blood of healthy humans and specific microRNAs accumulate in disease states such as cancer. To establish this new paradigm, we will focus on microRNAs secreted into the blood by cancer cells and use mouse models to determine whether the microRNAs are taken up by and influence distant organs and tissues. In addition, we will purify microRNAs in their natural state from blood and determine whether if re-injected into the bloodstream they can travel to specific target sites and have a hormone-like action. Establishing this new paradigm would have a major impact not only on basic understanding of human physiology but could also open up new ways of diagnosing and treating disease based on RNA hormones in the blood.